Facebook held a big press event this morning. The company only sent out invitations last week, and when it did, it didn’t disclose the topic. But in a sense, everyone who pays close attention to Facebook has been waiting for this event for most of the site’s history — because the subject was Graph Search, the social network’s first truly serious search feature.

Like other great big Facebook-altering features like Timeline, Graph Search isn’t being unleashed in one fell swoop. It’s a beta for now, and only a minuscule percentage of users will get it immediately. (Everyone else can get on the waitlist at facebook.com/graphsearch.) Still, its arrival in even limited form is an important moment.

Graph Search lets you enter plain-English concepts that tie together multiple things the social network knows about the people who use it: where they live, where they work, where they’ve taken photos and what things they’ve liked. Among the searches Facebook provides as examples are “people who like tennis and live nearby,” “tourist attractions in Italy visited by my friends,” “movies liked by people who are film directors” and “friends of friends who have been to Yosemite National Park.”

The results aren’t Google-style links to external sites — they’re Facebook pages for the people, places and other things that match the query. In theory, at least, this new way to find information is both faster and far more personal than a conventional search engine. The notion that we might end up doing most of our interacting with Facebook by searching rather than browsing seems entirely plausible.

At the press event, Facebook did try to keep everyone’s expectations in check. CEO Mark Zuckerberg spent much of his presentation emphasizing what Graph Search isn’t. He kept saying that it’s not the same thing as “Web search” (or, as most of us tend to call it, Google).

More important, it isn’t a fully fleshed-out idea. This first version helps you find people, photos, interests and places, but there’s lots of stuff on Facebook it doesn’t know about yet. It doesn’t search status updates, for instance, and it’s not hooked into Facebook’s Open Graph, which ties the site together with third-party services like Spotify. It also doesn’t work on Facebook’s mobile apps yet. (Zuckerberg says Facebook’s to-do list for Graph Search should keep the company busy for the next several years.)

It’s not even clear yet just how useful Graph Search’s limited initial features will be. As I scanned the results for the example searches performed by Zuck and his co-presenters, Graph Search honchos Tom Stocky and Lars Rasmussen, many of them seemed painfully obvious, like a list of friends’ favorite TV shows that contained “well, duh” items such as The Office and Game of Thrones.

The most instantly appealing aspect of Graph Search may be photo search, which looks slick and fun. Without it, I have trouble finding my own snapshots I’ve uploaded, let alone ones from friends or public ones I might like.

If Graph Search fails to come up with any results at all — an outcome that sounds like it might be common — it’ll give you a standard page of Web search results. Those will be powered by Facebook partner and part owner Microsoft, and will be largely the same as the results you’d get if you just conducted the same search on Bing.

When Facebook launches something major, it’s easy to get irrationally exuberant about its potential: the e-mail-like messaging features it introduced in November 2010, which looked like they might be a landmark, didn’t usher in a new era for either Facebook or e-mail after all. But it’s been clear all along that a sophisticated search feature was the biggest missing piece of the Facebook puzzle — the one to let the site tap into everything it knows about a billion people in ways that might be both highly useful and enormously profitable. Now we know that Facebook is at work on that challenge.

Other than allowing for “Sponsored Search” promotional links, Facebook isn’t trying to make money off Graph Search yet. But if it takes off, and people start using Facebook to find restaurants, doctors, travel destinations and much more, it’s tough to imagine a scenario in which the site doesn’t make enormous amounts of money from precisely targeted advertising. It could rival Google as a money machine on a level it hasn’t done to date.

We’re talking Facebook here, so it’s a given that Graph Search is going to be controversial, at least at first. It doesn’t let you see anything you couldn’t have seen otherwise: all privacy settings are respected, and people won’t even show up in results if they haven’t given you access to the relevant facts that would otherwise put them there, such as their employer or location.

But Graph Search will make it a whole lot easier to find out stuff on Facebook. Facts that would otherwise have been buried in a profile or that might have scrolled off into the Timeline’s distant past will be only a search query away. It’ll be fascinating to see how that changes the way people use the service, and whether it rankles a meaningful number of users.

Facebook isn’t introducing any new privacy features related to Graph Search; at today’s event, Zuckerberg simply recapped recent privacy upgrades like the ability to review all your recent activity and adjust settings on the fly. He also said before Graph Search is made widely available, Facebook members will get a prominent message encouraging them to review their privacy choices. I expect that this won’t be enough to pacify privacy advocates and Facebook skeptics — though truth to tell, I think that much of the angst over Facebook privacy issues is overblown. (I’m never clear on why people who dislike Facebook use it at all.)

Me, I’m guardedly optimistic about Graph Search. I use Facebook for things I want to share, not for things I want to hide, so I’m not worried about the privacy implications. And my single biggest beef with the site is that it’s impenetrable: too much of the worthwhile stuff it contains is too hard to uncover. Graph Search could change that.